1. Field of the Invention
The present invention relates to a method of acquiring physical information and a physical information acquiring device. More particularly, the invention relates to an operation mode in which partial information is used by reducing an information amount, by using a semiconductor device for physical quantity distribution detection, such as a solid-state imaging device including an array of a plurality of unit elements sensitive to an electromagnetic wave, such as light or radiation, incident from the outside and capable of reading an electrical signal indicating a physical quantity distribution converted into the electrical signal by the unit elements.
2. Description of the Related Art
In various applications, to detect a physical quantity distribution, a semiconductor device is widely used which includes a linear array or a matrix array of unit elements (for example, pixels) sensitive to a change in a physical quantity, such as pressure (contact or the like) or an electromagnetic wave such as light or radiation incident from the outside.
For example, in video apparatuses, a solid-state imaging device is used which includes an imaging device of a CCD (Charge Coupled Device) type, a MOS (Metal Oxide Semiconductor) type, or a CMOS (Complementary Metal-Oxide Semiconductor) type to detect a change in a physical quantity such as light (which is an example of an electromagnetic wave).
In computer apparatuses, a fingerprint recognition apparatus is used to acquire fingerprint information by detecting an image of a fingerprint based on a change in an electrical or optical characteristic corresponding to pressure. In these apparatuses, a physical quantity distribution is converted into an electrical signal by unit elements (pixels in the case of a solid-state imaging device) and the resultant electrical signal is read out.
Among the solid-state imaging devices, there is an amplification-type solid-state imaging devices. The amplification-type solid-state imaging device includes pixels of an amplification-type solid-state imaging device (Active Pixel Sensor (APS), also referred to as gain cell) in which a driving transistor for amplification is disposed in each pixel signal generating unit that generates a pixel signal corresponding to a signal charge generated by a charge generating unit. This structure is used in most of CMOS solid-state imaging devices.
In recent years, an XY address-type solid-state imaging device, for example, a CMOS image sensor is widely used which is better than a CCD-type image sensor in terms of reduction of a consumed power and a decrease in a system size.
In this amplification-type solid-state imaging device, to read out an image signal, addressing control is performed on a pixel array where a plurality of unit pixels are disposed, and signals are read from the respective unit pixels according to a determined address order or any order. That is, the amplification-type solid-state imaging device is an example of a solid-state imaging device of an address control type.
Further, in an amplification-type solid-state imaging device that is an example of an X-Y addressing solid-state imaging device in which unit pixels are arranged in a matrix, each pixel is configured by using an active element having a MOS structure (MOS transistor) so as to have an amplification capability. In this structure, a signal charge (photoelectron or hole) accumulated in a photodiode serving as a photoelectric conversion device is amplified by the active element and read out as image information.
In the X-Y addressing solid-state imaging device of this type, for example, a pixel array is formed using a large number of pixel transistors arranged in a two-dimensional matrix. Accumulation of signal charges corresponding to incident light is started on a line-by-line (row-by-row) basis or a pixel-by-pixel basis, and a current or voltage signal corresponding to the signal charge accumulated in each pixel is read sequentially from the respective pixels by accessing the pixels by means of addressing. In solid-state imaging devices of the MOS type (and of the CMOS type), the addressing is performed, for example, such that pixels are simultaneously accessed on a row-by-row basis and pixel signals are read from the accessed pixels, that is, pixel signals are read on a row-by-row basis from a pixel array.
<Structure and Operation of Solid-State Imaging Device According to Related Art>
FIG. 1 is a diagram illustrating a schematic structure of a CMOS solid-state imaging device (CMOS image sensor) according to the related art. A solid-state imaging device 1 shown in FIG. 1 is configured such that an amplifying transistor outputting a pixel signal from a unit pixel 3 forms a source follower circuit. In the same manner as JP-A-2001-298748, the transistors form a current mirror circuit.
As shown in FIG. 1, the solid-state imaging device includes an imaging unit 10 (pixel array) in which a plurality of unit pixels 3 are disposed, a scanning horizontal unit 12 and a vertical scanning unit 14 that are provided outside the imaging unit 10, a column processing unit 20 having column signal processing units 22 that are disposed for every column, a read current source unit 27 that supplies a pixel signal reading operation current (read current) to the unit pixels 3 of the imaging unit 10, a horizontal selection switch unit 60, and an output circuit 88. Each functional unit is provided on the same semiconductor substrate.
Although not shown in detail in the drawing, the unit pixels 3 are disposed in rows and columns, that is, in a two-dimensional matrix. Further, in order to select a predetermined row and read a pixel signal to a vertical signal line 18 (signal read line), each unit pixel 3 is connected to a row control line 15 that is controlled by a vertical scanning unit 14 or a vertical signal line 18 (output signal line) for transmitting a pixel signal to the column processing unit 20.
Although not shown in detail in the drawing, in the unit pixel 3, a photodiode or a pixel signal generating unit having a floating diffusion amplifier structure are provided. In this case, the photodiode generates a signal charge according to an amount of received light, and the pixel signal generating unit generates a pixel signal on the basis of the generated signal charge.
In this case, in the pixel signal generating unit, an amplifying transistor 42 having a source follower structure, which forms a circuit between the amplifying transistor 42 and the read current source unit 27 and is supplied with almost constant currents (a read current and a load current), is provided.
The column processing unit 20 is disposed on a signal path between a vertical signal line 18 of each column and a horizontal signal line 86 (locations where the column processing unit 20 is disposed are not limited to locations between the vertical signal line 18 and the horizontal signal line 86 in terms of a layout), receives a pixel signal from each vertical signal line 18, performs a predetermined signal process on the pixel signal, if necessary, and transmits a pixel signal of a predetermined column to the output circuit 88 through a horizontal signal line 86 by horizontal scanning using the horizontal scanning unit 12 (address selection in a horizontal direction).
For example, in the column processing unit 20, a circuit (column signal processing unit), which has an accumulating function holding a pixel signal S1 from a unit pixel 3, is provided. As an example, a switching transistor 112 and a storage unit 114 holding a voltage value according to the pixel signal S1 are provided. Further, the storage unit 114 may be used for a CDS (correlated double sampling) process function unit disclosed in Kazuya Yonemoto, “Basics and Applications of CCD/CMOS Image Sensor”, CQ Publishing Co., Ltd., Aug. 10, 2003, First edition, Chapter 6 (FIGS. 6 to 9).
A gate of the transistor 112 is supplied with a write control signal MWR that holds a pixel signal in each storage unit 114. The pixel signal S1 is output to the vertical signal line 18, in which a potential according to a signal charge outputted from the photodiode according to incident light is reduced by a threshold voltage value of the amplifying transistor 42 having a source follower structure, and is then applied to the storage unit 114.
A switching transistor 122 is provided in a horizontal selection switch unit 60. The switching transistor 122 is configured such that its gate is supplied with a column selection control pulse φg from the horizontal scanning unit 12, and it outputs a pixel signal voltage Vm (pixel signal S2) stored in the storage unit 114 to the horizontal signal line 86 side at a predetermined timing.
The read current source unit 27 includes a transistor 303 (in particular, referred to as load MOS transistor) provided for each column, and a reference current source unit 310 that has a current generating unit 312 and a transistor 314 and is used commonly for all columns. The load MOS transistor 303 of each column is connected to the transistor 314 of the reference current source unit 310 such that they form a current mirror circuit therebetween. The reference current source unit 310 functions as a current control unit that controls a current flowing through the load MOS transistor 303 (corresponding to a current source transistor 304 of FIG. 5 or the like) that is provided for each column.
The output terminal of each column signal processing unit (signal holding unit 24) is connected to the horizontal signal line 86 through a transistor 122 of the horizontal selection switch unit 60. The horizontal signal line 86 is connected to the output circuit 88. The imaging signal S3 that is output from the output circuit 88 is output to the outside of the solid-state imaging device.
In this case, in the related art, the image signal S1 is amplified by using a source follower, and the load MOS transistor 303 of a current mirror type serving as a load is disposed for each column. The current value of the load MOS transistor 303 is determined by the reference current source unit 310 that functions as the load MOS current source.
The storage unit 114 is provided for each column, which forms a signal holding unit 24 of a line memory structure. The pixel signals that correspond to one row are simultaneously read out to the storage units 114 and are then temporarily stored therein. Then, the pixel signal that corresponds to the selected column at a predetermined timing by means of horizontal scanning by the horizontal scanning unit 12 is read out to the horizontal signal line 86, and transmitted to the output circuit 88.
Meanwhile, in a technology for imaging a still image that is representative of a digital still camera, a solid-state imaging device having a plurality of pixels is used as an imaging device, and pixel information of all of the pixels are independently read out to form a still image. For example, in the digital still camera, since it is necessary to achieve high definition in a still image photographed by releasing a shutter, a solid-state imaging device having a large number of pixels is used.
Meanwhile, read of the pixel information is not limited to a case where pixel information of all of the pixels is independently read out, but an operation mode may be set in which an actual amount of used pixel information is reduced, and only a portion of pixel information of the pixel array is used. For example, a thinning-out read mode in which a pixel signal is read for each row at a predetermined interval or each column at a predetermined interval, or a division mode in which a region is divided and a read operation is performed for the divided regions may be used.
For example, before photographing a still image, generally, a moving picture (subject image) is reflected on a small screen, for example, a liquid crystal monitor to confirm (monitor) the subject. In a stage where the subject is confirmed (monitoring mode), it may be a rough image (low-resolution image) according to the number of pixels in a liquid crystal monitor.
Further, when transmitting an image in a portable apparatus, such as a digital still camera or the like, a data transmission rate is limited. Accordingly, as for a still image, pixel information of all of the pixels is transmitted so as to obtain a high-resolution image, and as for a moving picture, in a state where the pixel information is thinned out to reduce an information amount, the pixel information is transmitted.
In a process of thinning-out pixel information when using a CCD-type image sensor as an imaging device, for example, a method is used in which after reading pixel information of all of the pixels from the image sensor (pixel array and imaging unit), pixel information is thinned out by using an external signal processing system. Further, even in a color camera where a MOS image sensor is used as an imaging device, a thinning-out read method in the CCD-type image sensor is used.
From this reason, in a single-plate-type color camera where a CCD-type image sensor or a MOS image sensor is used as an imaging device, after pixel information of all of the pixels is read out from an imaging sensor, a process of thinning-out the pixel information is performed by using an external signal processing system. In this case, however, since a driving frequency of the image sensor does not vary regardless of decreasing an information amount by the thinning-out process, the power consumption is not reduced, but a load is applied to a signal processing system of a subsequent stage.
In particular, in the single-plate-type camera where the MOS image sensor is used as an imaging device, the CCD-type image sensor is advantageous in low power consumption and a small-sized system, as described above. Therefore, when an amount of pixel information that is actually used is reduced, if the power consumption can be reduced without applying a load to a signal processing system of a subsequent stage, the effect thereof is increased. As a system that meets these requirements, there is a system disclosed in JP-A-2001-298748.
In the system disclosed in JP-A-2001-298748, an XY address type solid-state imaging device where a color coding arrangement is used as a unit, the unit is repeated (for example, repetition of two verticals×two horizontals), and color filters having a predetermined color coding are disposed is used. In this case, when a thinning-out read mode is designated, a clock frequency of a system is converted into a low frequency according to a degree of thinning-out, pixels are selected while skipping several pixels-by-several pixels in a row direction and a column direction based on the converted clock frequency, and pixel signals are sequentially read out.